It is up to the UK to decide what action to take to cut PM10 levels in London. It could reduce traffic in congested areas where PM10 is above EU limits, or target construction work, ships at berth, industrial plants or domestic heating.

A spokesperson for London Mayor Boris Johnson welcomed the Commission's decision on Friday as "recognition of the serious steps that the mayor has taken to reduce PM10 pollution...

"We have robust, sensible and proportionate measures in place to reduce pollution levels."

The measures include a dust suppressant trial, encouraging low and zero emission vehicles, taxi age limits and a hydrogen-fuelled bus service.

EU infringement procedures have been opened against 20 of the EU's 27 member states over air quality.

Two countries - Slovenia and Sweden - have been referred to the European Court of Justice over their PM10 levels.